Use intersection observer to control frame throttling

third_party/WebKit/Source/core/dom/IntersectionObserver.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IntersectionObserver_h
 #define IntersectionObserver_h
 
 #include "bindings/core/v8/ExceptionStatePlaceholder.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/dom/IntersectionObservation.h"
@@ skipping 15 matching lines @@
 class CORE_EXPORT IntersectionObserver final
     : public GarbageCollectedFinalized<IntersectionObserver>,
       public ScriptWrappable {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
   using EventCallback =
       Function<void(const HeapVector<Member<IntersectionObserverEntry>>&),
                WTF::SameThreadAffinity>;
 
+  // Defines the assumed initial state of the observed element. If the actual
+  // state is the same as the initial state, then no observation will be
+  // delivered. kAuto means the initial observation will always get sent.
+  enum class InitialState {
+    // TODO(skyostil): Add support for kVisible.
+    kAuto,
+    kHidden,
+  };
+
   static IntersectionObserver* create(const IntersectionObserverInit&,
                                       IntersectionObserverCallback&,
                                       ExceptionState&);
   static IntersectionObserver* create(const Vector<Length>& rootMargin,
                                       const Vector<float>& thresholds,
                                       Document*,
                                       std::unique_ptr<EventCallback>,
                                       ExceptionState& = ASSERT_NO_EXCEPTION);
   static void resumeSuspendedObservers();
 
@@ skipping 19 matching lines @@
   void applyRootMargin(LayoutRect&) const;
   unsigned firstThresholdGreaterThan(float ratio) const;
   void deliver();
   void removeObservation(IntersectionObservation&);
   bool hasEntries() const { return m_entries.size(); }
   const HeapLinkedHashSet<WeakMember<IntersectionObservation>>& observations()
       const {
     return m_observations;
   }
 
+  // Set the assumed initial state of the observed element. Note that this can
+  // only be set before calling observe().
+  // TODO(skyostil): Move this setting to IntersectionObserverInit once the API
+  // is finalized.
+  void setInitialState(InitialState);
+
   DECLARE_TRACE();
 
  private:
   explicit IntersectionObserver(IntersectionObserverCallback&,
                                 Node&,
                                 const Vector<Length>& rootMargin,
                                 const Vector<float>& thresholds);
   void clearWeakMembers(Visitor*);
 
   Member<IntersectionObserverCallback> m_callback;
   WeakMember<Node> m_root;
   HeapLinkedHashSet<WeakMember<IntersectionObservation>> m_observations;
   HeapVector<Member<IntersectionObserverEntry>> m_entries;
   Vector<float> m_thresholds;
   Length m_topMargin;
   Length m_rightMargin;
   Length m_bottomMargin;
   Length m_leftMargin;
+  InitialState m_initialState;
 };
 
 }  // namespace blink
 
 #endif  // IntersectionObserver_h